The present invention concerns a shadow mask or color selection electrode for a television image tube or display (visualization) in color (cathoderay tube). It concerns, more particularly, means for attaching the mask to its support framework that are disposed in such a way as to reduce the initial distortion (the swelling or bulging or doming) of the mask under the effect of the heat generated by its electron bombardment.
A shadow-mask tube is generally formed of a glass envelope coprising a front panel (or plate) having a rectangular form, surrounded by a lateral wall in the form of a skirt, which is sealed at a part called the "conical" part which tapers and finishes with a tubular neck, housing at the end an assembly of three electron guns. Around this neck, horizontal and vertical electromagnetic deflectors are provided, allowing to carry out the sweep of the luminescent screen.
This screen, formed of luminescent material of three primary colors, red R, blue B, and green V, is deposited on the internal face of the plate. In a type of tube where the electron guns emit three parallel electron beams situated in a single horizontal plane, this screen is constituted by a repetitive succession of three vertical bands of luminescent material of three different colors R, V, B.
The color selection electrode is constituted by a metal surface provided with a great number of oblong (or elongated rectangular) openings; it is called a shadow-mask and is disposed on the path of the three electron beams adjacent to, and substantially parallel with, the screen. This mask has the effect of allowing to pass only that part of each electron beam which is directed towards a single band of luminescent material R, V and B, so that one beam is intended to hit the green bands, another beam only reaches the blue bands B and the last beam only bombards the red bands R; the selection is obtained due to the difference of the incident angles of the beams at the site of the slots. But the major part (about 80%) of the electrons of each beam hit the mask without crossing through the slots. This means that a rapid heating occurs at the part of the mask swept by the beams.
Since the mask must, during the manufacturing of the tube, be removed and put back into place several times and, furthermore, be capable of supporting predetermined mechanical shocks and vibrations, without undergoing permanent distortions or displacements, it is generally supported by means of a fixed metal frame which is, preferably, constituted by a profiled piece having an L-shaped cross-section and a thickness substantially greater than that of the mask (by 10 to 15 times, for example). The thickness of the mask is generally between 100 and 200 micrometers, and that of the frame between 2 and 3 millimeters. These values depend, of course, upon the dimensions of the screen.
Due to this fact, the thermal inertia of the frame is much higher than that of the mask; this frame is thus heated only much more slowly. Therefore, the shadow mask is, once the tube operates, heated much more quickly than the heavy and thick frame. This frame itself is attained only slightly by the electrons, which generally contact it only in the vicinity of the beginning and the end of each line sweep and each frame sweep. Consequently, it is mainly heated from the mask and it only reaches its equilibrium temperature much later than the mask. A swelling or bulging called "doming of the mask" is thus observed, the central part of which approaches the screen and the edges of which, being welded to the frame, are fixedly maintained in position by it. The frame is itself secured to the skirt of the front panel only by conventional assembly means having spring blades. This temporary swelling of the perforated mask causes displacements of the slots which, in the center, are exclusively axial; they present axial components decreasing from the center towards the periphery (where they are initially zero) and radial components which increase from the center (where they are zero) until about half-way between the center and the edge (where they reach their maximal values) and from there said components decrease towards this periphery. This situation is diagrammatically illustrated in the sectional view of FIG. 1, where a curve A in dashed line shows the profile of a cold mask 12 and a cold frame 16, while a curve B in mixed lines shows the profile of a hot mask 12 with a cold frame 16 causing the said swelling. The above-mentioned displacements of the slots have the effect of displacing the axes of those portions of the beams which cross through them with respect to the vertical axes of the bands of luminescent material R, V and B, associated in juxtaposed triplets, in such a way as to cause register losses, or alignment defects, that are the highest in an annular zone located about mid-way between the center and the edge of mask 12.
This can result in either a relative decrease of the luminous intensity substantially proportional to that of the surface of the bombarded luminescent material (if the bands are separated by phosphorus-free zones), or defects of color purity, since a beam intended for a single luminescent material hits partially an adjacent band of another color.
After a selected operating time of the tube, the frame 16 is also heated progressively, by conduction, by radiation and possibly by electron bombardment. Since the frame 16 and mask 12 are generally made of the same material (laminated steel), they present the same thermal expansion coefficient. The expansion of frame 16, resulting from that of mask 12, has the effect, on the one hand, of reducing its swelling (by flattening it with respect to the curve B of FIG. 1), and on the other hand, of increasing the shift between its slots, i.e. of displacing them radially. This is diagrammatically illustrated in the sectional view in FIG. 2, which shows (curve A in dashed line, analogous to that of FIG. 1) the profile of a cold mask-plus-frame assembly and (curve C in full line) a hot mask-plus-frame assembly, i.e. a mask and frame having reached a same equilibrium temperature. It is observed that the size of mask 12 as well as the shift between the pairs of parallel arms of frame 16 have increased and that the radius of curvature of mask 12, after a brief reduction due to the initial swelling, becomes slightly higher than that which had prevailed in the cold state. If frame 16 is suspended solely by using spring blades, the longitudinal axes of which are positioned in a single median (transversal) plane and are substantially tangential with respect to its circumference, frame 16 can expand in its plane without undergoing any axial displacement. This has the effect of stretching the surface of the mask so that it spreads out by flattening slightly. Mask 12 thus undergoes a slight axial displacement at the center which increases with the radial distance, and a spreading out in the radial direction which has the effect of producing an increase of the spacing of the slots and, to a lesser extent, an increase of their width. This results in register losses due to the spreading out of the slots in the plane of the expanded surface, which increase with their radial distance with respect to the axis of the tube (i.e. with respect to the center of the mask 12). It has been determined that a supplementary displacement of the hot mask 12-plus-frame 16 assembly (profile C) in the direction of the screen by following the axis of the tube, allowed to compensate these register losses, such a displacement allowing substantially to maintain the center of curvature of the surface of mask 12 at the intersection of the axis of the tube with the deviation plane perpendicular to this axis. This axial displacement towards the fore, illustrated by profile D (without frame) in FIG. 2, is obtained either by blade springs (cf. for example, French Pat. No. 1 540 869), or by using bimetallic strips components inserted between one end of the blade spring and frame 16. However these bimetallic compensation elements are not involved during the initial swelling of mask 12. This swelling can be especially reduced, as well as other distortion effects exerted on the edges of mask 12, by limiting the number of welding sites or sealings joining the skirt of the mask 12 to the belt of frame 16 that are parallel, as disclosed in French pat. No. 1 470 260.
Various arrangements have allowed supplementary reductions of initial swelling to be achieved both in amplitude and duration. In particular, it is possible with this purpose, to utilize a frame of reduced thickness, strengthened by at least one rib or fold in order to present sufficient mechanical strength. It is also possible, and this is the system used in the present invention, to use bimetallic strips as intermediary attachment means between the mask and the lateral wall of a frame.
But this known solution does not, as such, reduce the temporary swelling sufficiently for certain applications- such as the display in computers (for example, videography) or in high definition television. In fact, these tubes present screens with finer luminescent material bands and masks with slots spaced closer together (interval reduced from 0.8 to 0.5 millimeters, for example) than in current tubes, and consequently which have much more tighter tolerances on the radial displacements of the slots. It will also be noted that if the radius of curvature is increased, so that it is flatter, the register loss is also increased due to the swelling. It is well understood that the requirements of temporary swelling reduction of the mask further increase for a flat screen of great resolution.